1. Field of the Invention
The present invention relates to an insulated gate field effect transistor and a tuning circuit.
2. Description of the Prior Art
LC tuned circuits are widely used in RF circuits, such as oscillators and tuned amplifiers, it is often necessary, for instance in multiband radio equipment, to change the resonant frequency of a circuit. At HF frequencies, relays can be used to switch capacitors, inductors or entire tuned circuits into and out of use. However, at higher frequencies, relays become unsuitable. Furthermore, relays have a relatively high current demand and are undesirable at any frequency for this reason.
Subsequently, PIN diodes came to be used for RF switching, including the switching of capacitors and inductors into and out of tuned circuits. However, in order for a PIN diode switch to close, the PIN diode must be forward biased which results in a significant current flow.
The use of MOSFETs to effect switched tuning has the advantage of negligible current demand and has been proposed in Kral, A. et al, “RF-CMOS Oscillators with Switched Tuning”, Proceedings of the Custom Integrated Circuits Conference, pp 555–558, 1998, Kuhn, W. et al., “A 200 MHz CMOS Q-Enhanced LC Bandpass Filter”, IEEE Journal of Solid-state Circuits, Vol. 31, no. 8, pp 1112–1122, August 1996 and Cho, T. et al., “A Single-Chip CMOS Direct-Conversion Transceiver for 900 MHz Spread-Spectrum Digital Cordless Phones”, Proceedings of the IEEE International Solid-State Circuits Conference, pp 228–229 & 464, 1999. However, the use of MOSFETs in this way has not become commonplace.
Furthermore, the known circuits are restricted to switching components in an out of circuit by controlling the resistance between a node, e.g. one terminal of a capacitor, and an AC ground, typically 0V for NMOS devices and +V for PMOS devices.